When oils and fats or articles of food containing oils and fats are oxidized, unpleasant odors are generated and peroxides are formed which exert undesirable effects on the human body. In addition, although fatty acids and their derivatives obtained from of animal or plant sources oils and fats are used in many industrial fields because of their surface-modifying functions, lubrication functions and physiological activities, fatty acids, especially unsaturated fatty acids, tend to undergo oxidation which causes coloring of fatty acids and the formation of peroxides that generate unpleasant odors. Fatty acids and their derivatives changed in quality by degradation of such peroxides are not desirable as materials for food, cosmetics, pharmaceutical drugs and the like.
Antioxidants are generally used to prevent oxidation of oils arid fats or fatty acids. In an oxygen-contacting system, however, the oxidation inhibiting effect does not last long because of rapid consumption of the antioxidant. Addition of an antioxidant in a large quantity (500 ppm or more) to strengthen its oxidation inhibiting effect is not desirable because it causes side reactions. For example, in the process of soap production, soap often develops an undesirable color when prepared by saponification.
Consequently, in order to minimize the amount of antioxidant while simultaneously improving the oxidation inhibiting effect, concurrent use of a synergist is generally practiced. Examples of synergists having an oxidation inhibition function include oxyacids, phosphoric acid and derivatives thereof, various amino acids and derivatives thereof, flavone derivatives, sulfur compounds and the like, of which oxyacids are most widely used.
German Patent Publication No. 2,038,468 discloses a process for the purification of fatty acids in which oxyacids are added to fatty acids and then distilled. This process, however, is insufficient to inhibit the oxidation of fatty acids. Also, because they have a low solubility in oils and fats or fatty acids, oxyacids are generally added in the form of aqueous or alcohol solution to oils and fats or fatty acids, water or alcohol is sufficiently removed by distillation under reduced pressure and then excess crystalized oxyacids are removed. This method, however, is not advantageous from an industrial point of view due to the reduction in quality of oils and fats or fatty acids caused by the solvent remaining after distillation, as well as the considerable time and labor required for carrying out these operation steps.
In addition, the use of ester derivatives of oxyacids with monoglycerides has been proposed and practiced with the aim of increasing their solubilities in oils and fats or fatty acids. However, the effects of such ester derivatives are insufficient and they are expensive as compared to oxyacids. Techniques for dispersing oxyacid solutions making use of surface active agents and the like have also been practiced. However, such surface active agents, solvents and the like become impurities in the oils and fats or fatty acids and reduce the quality of the resulting products.